MUSCLE-ACTIVITY ADAPTS TO ANTIGRAVITY POSTURE DURING PEDALING IN PERSONS WITH POSTSTROKE HEMIPLEGIA

With hemiplegia following stroke, a person's movement response to anti -gravity posture often appears rigid and inflexible, exacerbating the motor dysfunction. A major determinant of pathological movement in ant i-gravity postures is the failure to adapt muscle-activity patterns au tomatically to changes in posture. The aim of the present study was to determine whether the impaired motor performance observed when person s with hemiplegia pedal in a horizontal position is exacerbated at mor e vertical antigravity body orientations. Twelve healthy elderly subje cts and 17 subjects with chronic (>6 months) post-stroke hemiplegia pa rticipated in the study. Subjects pedalled a modified ergometer at dif ferent body orientations (from horizontal to vertical), maintaining th e same workload, cadence, and hip and knee kinematics. Pedal reaction forces, and crank and pedal kinematics, were measured and then used to calculate the work done by each leg and their net positive and negati ve components. The EMG was recorded from four leg muscles (tibialis an terior medial gastrocnemius, rectus femoris and biceps femoris). The m ain result from this study was that impaired plegic leg performance, a s measured by net negative work done by the plegic leg and abnormal ea rly rectus femoris activity, was exacerbated at the most vertical body orientations. However contrary to the belief that muscle activity can not adapt to anti-gravity postures, net positive work increased approp riately and EMG activity in all muscles showed modulated levels of act ivity similar to those in elderly control subjects. These results supp ort the hypothesis that increased verticality exacerbates the already impaired movement performance. Yet, much of the motor response to vert icality was flexible and appropriate, given the mechanics of the task.